This invention relates to an electro-optical element employing nematic liquid crystal materials of a positive dielectric anisotropy.
It is known that nematic liquid crystals (hereinafter referred to as "N-liquid crystals") can be used for display, light modulation and the like by utilizing their specific property that optical characteristics are changed under application of electric fields, magnetic fields, ultrasonic waves and the like. Such elements comprise, in general, an N-liquid crystal filled and supported between two substrates disposed to confront each other with a distance smaller than about 50.mu., at least one of which is transparent, and the change in the molecular arrangement caused under application of electric fields, magnetic fields, ultrasonic waves and the like is utilized for light modulation.
Compounds forming such N-liquid crystal are divided into two types depending on the molecular structure and dielectric properties, one type being characterized in that the molecular axis and electric dipole are substantially vertical to each other (the liquid crystal of this type will hereinafter be referred to as "Nn-liquid crystal") and the other being characterized in that the molecular axis and electric dipole are substantially in parallel (the liquid crystal of this type will hereinafter be referred to as "Np-liquid crystal"). Accordingly, the Nn-liquid crystal indicates an N-liquid crystal having a negative dielectric anisotropy, and the Np-liquid crystal indicates an N-liquid crystal having a positive dielectric anisotropy.
The conventional Np-liquid crystal electro-optical elements comprises a pair of electrode plates disposed to confront each other and a Np-liquid crystal layer interposed between said plates. The molecular axes of the Np-liquid crystal are made parallel with the electrode faces and are arranged in the substantially same direction in a plane parallel to the electrode plate. If seen from the direction vertical to the electrode plate, molecular axes are arranged in the state continuously distorted from one another between adjacent faces. Such an orientation of the molecular axes is prepared by rubbing the electrode face along one direction with fabrics, paper or the like and piling the two rubbed electrodes in such a way that the rubbing directions are at right angles to each other. The molecular axes near by the electrode face are oriented along the rubbing direction while the molecular axes in the liquid crystal layer are oriented in the state continuously distorted. When polarized light passes through this liquid crystal layer, the polarization plane of the light is rotated depending on the degree of distortion of the molecular axis direction. This distortion of the molecular axis direction can be relaxed by application of an appropriate electric field. Accordingly, by adjusting the intensity of the electric field it is made possible to adjust rotation of the polarization plane of polarized light passing through the element.
When the Np-liquid crystal element is interposed between two polarizing plates, it changes from the light-shielding state to the light-transmitting state or from the light-transmitting state to the light-shielding state depending on the applied voltage, and this light modulation is utilized for display.
Since the light modulation process utilizing the Np-liquid crystal element provided between two polarizing plates disposed in such a way that the oscillation planes of light cross to each other exhibits a mechanism quite different from a Nn-liquid crystal electro-optical element, a higher contrast ratio can be obtained in use for display of a pattern. Further, the Np-liquid electro-optical element can be utilized for construction of a Boolean algebra generator, a logical product gate, a "nor" gate and a more complicated logical circuit. If an Np-liquid crystal electro-optical element capable of responding quickly is employed, it can be utilized for display of a three-dimensional television or moving picture. Further, in the case of the Np-liquid crystal electro-optical element, since the wavelength region of rays of transmission varies depending on the electric voltage when it changes within the range of the threshold value to the saturation voltage, it can be used for display of colors.
In the conventional Np-liquid crystal electro-optical elements, the process of rubbing the electrode face directly in one direction is employed for orientation of the liquid crystal molecules as mentioned hereinbefore. With this process, however, it is impossible to orient the liquid crystal molecules uniformly so that the display in the obtained element becomes nonuniform and can not exhibit the sufficient properties. By way of the example, in a light-modulation system where the element in which the rubbing directions are at right angles to each other is provided between two polarizing plates of which polarizing axes are parallel each other, it should shut off light to render the display black under no application of electric field, but as a matter of fact, the display appears dark grey because the element transmits more or less light due to nonuniform orientation of the liquid crystal molecules. Further, since a percent transmission in uneven in part, the display has the bright and dark parts mixed together. This becomes serious obstacle for practical uses.
Moreover, since the liquid crystal material comes into contact with the electrode face and therefore, electro-chemical reaction takes place at the inter-face so that the life of the element becomes short.
As for means for orientation of the Np-liquid crystal molecule, in addition to the process of rubbing the electrode face in one direction, the process of providing an alignment film on the electrode face is disclosed in U.S. Patent application Ser. No. 242,675 wherein platinum, gold, tin, lead, aluminum, copper, silver, silicon monoxide or chromium are applied on the electrode face in the form of an alignment film by an oblique incidence deposition. Also, U.S. Pat. No. 3,700,306 discloses the electrode film protected with a thin overcoat of sintered or fused glass or silicon oxide to prevent the electro-chemical reaction at the interface between the liquid crystal layer and the electrode face and accordingly, to prolong the life of the element.